This invention relates to an inductive position sensor, and more particularly to inductive position sensors having a relatively movable scale and reading head, the scale comprising a spatially periodic series of conducting or permeable features of spatial period T, and the reading head comprising drive and sense windings facing the scale with a spatially periodic configuration of spatial period T2 along the scale.
In such sensors, the signal coupled from one winding to another via the scale's spatially periodic features varies sinusoidally with the reading head's position along the scale, its spatial period being equal to the scale's spatial period T. Measuring two or more such signals yields two or more mutually shifted sinusoidal functions, from which the reading head's position along the scale may be determined. Such sensors are simple, rugged and compact. External shielding is usually unnecessary, as their multi-polar windings are hardly sensitive to external fields and do not generate appreciable far fields themselves. If required, though, a printed circuit copper layer provides adequate shielding at the high frequencies used with such low inductance windings.
A first example of such a sensor is disclosed in U.S. Pat. No. 5,804,963 to Meyer, the entire contents of which are incorporated herein by reference. All windings, whether inducing (drive windings) or induced (sense windings), are interlaced in the same area facing the scale's full width, and all have the same meander shape with a full zigzag spatial period T2, i.e. twice the scale's spatial period T. In this embodiment all scales work, notably the simpler ones based on eddy currents or on permeability, such as conductive or ferromagnetic gears and racks. Unfortunately, uneven direct coupling between interlaced windings sharing the same magnetic field creates measuring distortions. These worsen if the gap between scale and reading head increases, as it decreases coupling via the scale, but not direct coupling.
A second example of such a sensor is disclosed in U.S. Pat. No. 7,015,687 to Meyer, the entire contents of which are incorporated herein by reference. All windings are also meander-shaped, with a full zigzag spatial period T2, but the interlaced drive windings are separate from the interlaced sense windings. As they occupy separate areas, direct magnetic coupling between them is strongly reduced, so that coupling via the scale by means of closed conductor loops becomes predominant. This winding configuration is thus less sensitive to uneven direct coupling than the first embodiment even though some direct coupling remains between separate meander shaped windings.